The invention relates to a method for the calculation of multilayer holograms, a method for the production of multilayer holograms as well as a holographic storage medium with a multilayer hologram produced according to this method. With multilayer holograms, it concerns in particular so-called computer-generated multilayer holograms. The latter exhibit special properties which can be used in numerous applications, in particular as security features. The properties are, in particular, angular selectivity, wavelength selectivity, blazing (only one order is visible), phase selectivity and others.
Various experimental techniques are known from the prior art of producing holograms. Mention may be made, on the one hand, of experimentally generated multilayer holograms and, on the other hand, of volume holograms.
Experimentally recorded two-layer holograms are known from the article from Optica Acta, 1986, Vol. 33, No. 3, pp 255-268. For these, two thin photosensitive layers (8 μm) are deposited on the opposite sides of a thick glass plate (1.34 mm). This glass plate is then illuminated with two plane waves which are at a certain angle to one another. The developed hologram is illuminated with readout waves which are varied both in angle as well as in wavelength. The theory developed in this article is based on splitting up a sine grating into Bessel functions and is in very good agreement with experiment. The two-layer holograms thus produced exhibit a higher diffraction efficiency than is permitted by the theoretical limit for single-layer holograms. They exhibit a high angular and wavelength selectivity, which however is periodic. They do not exhibit any blazing effect, i.e. positive and negative orders are of the same order of magnitude in intensity. As an application, an analog-to-digital converter is presented, which relies above all one of the property of angular selectivity.
Volume holograms display many properties that can be exploited in applications. Hitherto, however, they can neither be calculated closed, nor is there any possibility known for exposing a calculated volume hologram into a voluminous carrier material. For this reason, they have to be exposed experimentally, which involves all the drawbacks of experimental holography.
Thin computer-generated holograms can be calculated in a known manner and exposed in a simple fashion, but they do not possess important properties that are known with volume holograms.
Multilayer holograms exhibit very similar properties to volume holograms. They can be exposed in a similar manner to thin holograms. Hitherto, however, there has not been a suitable method for calculating such holograms.